An orthogonal frequency division multiple access system (hereinafter, referred to as “OFDMA”) implements multiple access by providing a portion of subcarriers usable in OFDM, which is a special type of multicarrier transmission, to each user. A basic principle of the OFDM divides data streams having a high transmission rate into a large number of data streams having a low transmission rate and simultaneously transmits the data streams using a plurality of subcarriers. Symbol duration of the subcarriers having the low transmission rate is increased, such that relative signal dispersion is reduced over time occurring due to multipath delay spread. Inter-symbol interference may be removed by inserting a longer guard interval than delay spread of a channel between the OFDM symbols and when replicating a portion of OFDM signals in the guard interval and disposing the replicated OFDM signal at a start portion of the symbol, the OFDM symbol is cyclically expanded to avoid inter-carrier interference.
The existing wideband code division multiple access system (hereinafter, referred to “WCDMA”) does not cause any problem even though it is operated without synchronizing between the base station and the user.
However, in the OFDMA environment, time synchronization of transmitting and receiving ends has a great effect on performance of the system. In the OFDMA environment, when the time synchronization of the transmitting and receiving ends are not performed, the orthogonality between respective subcarriers is lost, thereby deteriorating the performance of the system. Meanwhile, in the mobile communication environment, when there is a shadow area in the room or the existing macrocell in the state where the exiting macrocell base station is present, a need exists for a base station having low power and short coverage in order to solve the problem. The base station is referred to as a femtocell base station.
When the femtocell base station uses the same frequency band as the macrocell base station, there may be a problem in that the orthogonality between the OFDMA subcarriers are lost due to the difference in the receiving and transmitting timing between the femtocell terminal and the macrocell base station.
FIG. 1 is a conceptual diagram for describing a general process of synchronizing between a base station and a terminal at a downlink and an uplink in an OFDMA environment.
Referring to FIG. 1, terminal 1 102 performs time synchronization of the downlink based on a timing when the terminal 1 receives a preamble signal P from a base station 101. In addition, the terminal 1 102 transmits a ranging signal R to the base station 101 and the base station 101 measures the ranging signal to transmit uplink synchronization information S to the terminal 1 102, thereby performing the time synchronization of the uplink.
FIG. 2 is a conceptual diagram for describing interference occurring due to inter-carrier asynchronization at the downlink when the femtocell is present in the macrocell region. In FIGS. 2 and 3, CP implies Cyclic Prefix replicating a portion of an OFDM signal.
A femtocell 210 is present at any position within a macrocell 200.
A terminal 1 202 communicates with a macrocell base station 201 and a terminal 2 212 communicates with a femtocell base station 211. In this case, the terminal 1 202 and the terminal 2 212 use different subcarriers and the same subcarriers having the same frequency band to communicate with each corresponding base station.
In this case, the terminal 1 202 may receive a signal 220 transmitted from the macrocell base station 201 and the terminal 2 212 may also receive a signal 221 transmitted from the macrocell base station 201 in addition to a signal 222 transmitted from the femtocell base station 211 and the interference of the signal 221 transmitted from the macrocell base station 201 should be removed. However, when the time synchronization between both signals 221 and 222 is not performed (represented by area A), there are problems in that the inter-carrier orthogonality is lost and the inter-carrier interference occurs.
The terminal 2 212 may receive a signal 230 transmitted from the femtocell base station 211. In addition, the terminal 1 202 may also receive a signal 231 transmitted from the femtocell base station 211 in addition to a signal 232 transmitted from the macrocell base station 201 and the interference of the signal 231 transmitted from the femtocell base station 211 should be removed. However, when the time synchronization between both signals 231 and 232 is not performed (represented by area B), there are problems in that the inter-carrier orthogonality is lost and the inter-carrier interference occurs.
FIG. 3 is a conceptual diagram for describing interference occurring due to inter-carrier asynchronization at the uplink when the femtocell is present in the macrocell region.
Similarly, a femtocell 310 is present at any position within a macrocell 300.
The macrocell base station 301 may receive a signal 320 transmitted from terminal 2 312 in addition to a signal 321 transmitted from terminal 1 302 and the interference of the signal 320 transmitted from the terminal 2 312 should be removed. However, when the time synchronization between both signals 320 and 321 is not performed (represented by area C), there are problems in that the inter-carrier orthogonality is lost and the inter-carrier interference occurs.
In addition, the femtocell base station 311 may also receive a signal 330 transmitted from the terminal 1 302 in addition to a signal 331 transmitted from the terminal 2 312 and the interference of the signal 330 transmitted from the terminal 1 302 should be removed. However, when the time synchronization between both signals 330 and 331 is not performed (represented by area D), there are problems in that the inter-carrier orthogonality is lost and the inter-carrier interference occurs.